http://www.telegraph.co.uk/active/9124298/Warm-up-your-greenhouse.html

Warm up your greenhouses

As you prepare to jump-start your garden, use tricks that protect your plants and save fuel at the same time, says Bunny Guinness


[Photo: Digital Vision/Getty

By Bunny Guinness
5:07PM GMT 05 Mar 2012

Burning furniture to keep your greenhouse warm could be seen as rather extreme. But when last year’s big freeze coincided with an oil crisis, specialist herb grower Jekka McVicar had to sacrifice all her house and oven heating over Christmas to keep her precious stock alive in the greenhouse until the oil tanker could get to her in rural South Gloucestershire to refuel. As the greenhouse plants in question were for my M & G Chelsea garden, I was exceptionally grateful.

The prolonged icy winters over the past three years have meant that keeping a greenhouse even frost-free has been quite a cost. Now, just as we want to jump-start the garden and get lots of early plants going, it makes sense to use canny tricks to save on fuel.

For those putting in a greenhouse, an ideal spot is against a south wall where it can bask in the radiated heat at night. The east-west alignment will maximise the amount of sunshine, too. If, like me, you’re stuck with an old greenhouse on a north-facing wall, don’t despair. Paint the outside wall white to reflect extra light and it will benefit from radiated heat. Mine grows much better tomatoes and cucumbers than you would think. My watercress loves it too!

The old-style pit greenhouses which are sunken about 70cm to a metre below ground level apparently really do save on heating. A canny builder client of mine built himself one and I copied him for my second greenhouse. The speed at which you can excavate the area with a mini digger and the fact that you can get away with concrete blocks (as opposed to bricks/stone) for the surrounding walls as they are not seen externally, probably means the difference in construction costs is fairly negligible.

Even if you are not sinking it, building lowish brick or stone sides conserves heat. Timber framework as opposed to metal reduces heat loss.

If you are looking to reduce running costs on an existing greenhouse, it makes sense to divide it up and only to heat the area you have to. Much of the heat is lost in the apex and many commercial growers pull over an insulated cover of some sort with a silvered underside to span the building at the eves during the night. You could consider putting a twin wall polycarbonate sheet across (light and easy to fix) and leave it in situ for colder periods.

In Germany, where growers are very energy conscious, many of their greenhouses are made entirely from twin-wall polycarbonate, although it has poorer light transmission – its heat loss coefficient (U value) is 0.6. (glass is 1.1 and 50mm polystyrene board is 0.1.) In a pit greenhouse you can fix the polycarbonate on top of the retaining walls for lower-growing plants. Filling all draughts with putty or draught-proofing strips is a must, and even sheltering the structure with a fence or hedge helps protect it from the prevailing wind. At night and in cold periods, adding extra duvets over plants in the form of thick fleece or bubble wrap will help save a few more degrees.

Trying to get some extra free heat can more than just tip the balance. Water barrels painted black, earth or stone floors will heat up by day and radiate heat at night. Making a compost bin, as they do in the polytunnel at the Centre for Alternative Technology (cat.org.uk), can help too.

Chloe Ward, display gardener from CAT, said they made rat-proof wire cages 1.5m (5ft) x 1.8m (6ft) x 1.2m (4ft) and put about 800mm (2.6ft) of straw in the base. They then add 100mm (4in) layers of alternating green waste and straw, making sure to add 200mm (8in) of new material each week to keep it going. They start filling in January and as they take about three weeks to heat up, are ideal for generating gentle heat to help vulnerable seedlings in early spring. You could make a similar system using a large builder’s bag: free heat and then compost.

Jack Strawbridge, an eco engineer and author of Practical Self-Sufficiency, says he does not have time to do that. Instead he made a heat sump in his greenhouse, detailed in his book. He dug a hole with a volume of one cubic metre and put in a series of plumbing pipes to allow the air to circulate and then filled it with broken, recycled glass. A fan (a small, solar-charged one from a computer) sucks air from the apex and blows the warm air through the glass during the day. The glass is heated up during the day and the fan reverses the process at night, giving heat out. They have had frosts of 21F (-6C) outside and inside it is always frost free. They have salads all winter and aubergines in February, but there is also a south-facing rear wall which no doubt helps too.

After last winter, I am biting the bullet and putting electricity and heating into my greenhouses. Having two greenhouses sounds flash, but both are home-made using recycled windows. The most efficient heaters are thermostatically controlled propane ones, though these give off water as well as extra CO2, which can exacerbate fungal problems. Additionally, the CO2 output is too much for orchids and citrus. The latter will quickly defoliate.

Alfred Brusius of Green Ideas (biogreen.de) sells propane, paraffin and electric heaters and he reckons the running costs of their propane heater (Black Forest, £199) is about 15 per cent cheaper than his most efficient electric one (Phoenix, £199 from amazon.co.uk). The latter is most definitely the most popular. It is highly efficient to run as it has a precise thermostat that can be set so it comes on just at 32F (0C). A lot of the cheaper ones will frequently heat when unnecessary and not heat when necessary. It was highly recommended to me by my Gardeners’ Question Time colleague Anne Swithinbank (whose husband trialled different ones). Even though Alfred’s paraffin heaters are of a new generation, they still emit messy carbon and a lot of moisture. As they have no thermostat they are the most expensive to run — around 50 per cent more than propane.

This weekend I am sowing peas, beans, cosmos, lettuce, tomatoes, aubergines, leeks, cleomes, beetroot, carrots and more. If we get a late frosty snap, I want to sleep soundly in my bed, knowing that they are snug too.

'Practical Self-Sufficiency’ by Dick Strawbridge and Jack Strawbridge (Dorling Kindersley, RRP £20) is available from Telegraph Books at £18 + £1.25 p & p. Call 0844 871 1516 or visit books.telegraph.co.uk

I have not heard that propane -CO2 will makes citrus leaves drop off. Electricity sounds much more expensive to me. Propane must be ok with some venting. I can't imagine Millet using electricity all winter in Colorado but I maybe wrong....Propane lit fake gas logs in fireplace that I have don't have to be vented according to instructions but please check your own instructions for your own heaters before you try to get by without venting. That can be deadly to people. Tom

Tom, my greenhouse is heated by a 250,000 BTU vented propane heater, backed up by a second 200,000 BTU vented propane heater, which is again backed up by two 100,000 infrared heaters that operate off a pilot light. All this is yet still backed up by a used 100 gallon thermostat controlled heated dye vat. I also use the dye vat as heated propagating table. All this is because in the area where I live, we experience a hard freeze every night, plus hard freezes many days from approximately the 1st of November to the middle of March. In fact here in Colorado the month of March is the snowiest month of the year. I have 6 in ground trees and 45+- container trees plus all kinds of other plants with no CO2/moisture problem. As in the link above I also use double walled polycarbonate glazing as side and end walls, and the roof is made of double air inflated polyethylene glazing. Then on top of all this, I use 100 X 55-gallon black water filled plastic drums as solar heated greenhouse benches. Even in cold Colorado, I doubt I will ever lose my crops from a frozen greenhous . - Millet (320-)

I think I know what "flash" means in earlier post here but Millet's green house is "flash fantastic". I still don't know why some propane gas fake logs in fireplaces are not vented. I know your are not suppose to use a propane grill for heat inside a home and I think natural gas fake logs always have to be vented....hate losing all that heat in the vent but got to be safe !

L B White heaters are not vented and they are used in chicken houses..Atlas tried them but had problems with warrenties. I personally like them but had problems with the oxygen sensor..

Most all greenhouse heaters, and EVERY heater in a commercial greenhouse, without exception, are vented heaters. Vented heaters, whether natural gas or propane heaters, eliminate the chance of ethylene gas being injected into the greenhouse. It is not the CO2, and moisture that is given off by a heater's combustion that causes a problem. CO2 and moisture are beneficial to greenhouse plants. Ethylene gas which is given off by non-vented heaters, and even from ethylene "safe" non vented heaters that are not keep in good condition, is what causes the problem. Ethylene toxicity in a greenhouse will defoliate many plants, and most especially citrus trees, very quickly. If ethylene toxicity is not quickly correct it will kill most greenhouse plants. - Millet

Millet I am not arguing with you I am just saying that we did use non vented heaters for a while as an option mostly to farmers growing tobacco transplants. The first heater that we had a problem with was owned be the present owner of Atlas. The L B Wtite heater is used in chicken houses. And call me cheap but I never vented my Modine heaters when I heated with gas.
One other thing, think of how many homes have gas stoves and house plants. I personally have instsllen non vented heaters in greenhouses that were ordered from Atlas. I will admit, the first time I saw them I was amazed that they were not vented.

Hershell, today there are non-vented heaters that burn so efficiently that ethylene is not a problem with them. They have been around for several years. They work fine as long as they are kept clean and running in good condition. If not taken care of properly, that when the problem happens. I used to work for a chemical company that sold chemicals and fertilizer to greenhouses. Therefore, I have been inside just about every commercial greenhouse in Colorado, plus many in Southern California. I NEVER ever seen even one non-vented heater in any commercial greenhouse. anywhere. The risk is just to high. Happy New Year to you and your family. - Millet

Just google it LBWhite greenhouse heaters and you can see something that you have NEVER seen before.

Hershell, I did Google LB White. After seeing their heater on their web site, I remembered reading their add in the a monthly magazine I subscribe to called "Greenhouse Grower", which is an information source magazine for commercial greenhouse operations. - Millet